This invention relates to conductors, and, more particularly, to compositions useful for producing electrodes in multilayer capacitors.
Multilayer monolithic capacitors comprise a multiple number of dielectric layers, at least some of which bear metallizations (electrodes) in desired patterns. Such capacitors are made from green (unfired) tape of ceramic particles held together with an organic binder, by cutting pieces of tape from a sheet of tape, metallizing some of the tape pieces, stacking and laminating the pieces of tape, and firing the resultant laminate to drive off any organic binders and vehicles and form a sintered (coherent) body, which is termed "monolithic."
Rodrieguez et al. U.S. Pat. No. 3,456,313 discloses a process for making multilayer capacitors. FIG. 1 of Fabricius U.S. Pat. No. 3,223,905 shows a multilayer capacitor, which may be of alternating palladium and barium titanate layers.
Metallizations useful in producing conductors for multilayer capacitors normally comprise finely divided metal particles, applied to dielectric substrates in the form of a dispersion of such particles in an inert liquid vehicle. Selection of the composition of the metal particles is based on a compromise of cost and performance. Performance normally requires the use of the noble metals, due to their relative inertness during firing on dielectric substrates to produce electrically continuous conductors, since base metals often are oxidized in air at elevated temperatures and/or react with the dielectric substrate during firing.
Also used often as electrodes in multilayer capacitors are the coprecipitated noble metal alloys of Hoffman U.S. Pat. Nos. 3,385,799 and 3,390,981 and Short U.S. Pat. No. 3,620,714.
It is known that the particle size in gross sintered noble metal objects, formed by compacting and sintering noble metal powder compositions, can be controlled. Such control is accomplished by adding to the powder composition, prior to sintering, refractory oxides such as thoria (Smithells U.S. Pat. No. 2,406,172); metals capable of forming non-volatile stable oxides (Streicher U.S. Pat. No. 2,636,819); refractory oxide particles of certain particle sizes such as the 13 oxides mentioned at column 3, lines 26-41 in Alexander et al. U.S. Pat. No. 2,972,529 and the 11 mentioned at column 3, lines 56-67 of Alexander et al. U.S. Pat. No. 3,180,727.
The use of thoria in the formation of gross sintered objects of base metals is disclosed in Gulamiche U.S. Pat. No. 3,515,523, where powders of iron, cobalt and/or nickel are mixed with chromium and thoria, and then agglomerated and sintered in a halogenated atmosphere. None of these references suggests dispersions of metal plus oxide in a printing vehicle, nor makes any reference to thick-film electrodes.
There exists a need for screen-printable compositions useful in forming capacitor electrodes which are less expensive than conventional noble metal systems and which can produce enhanced capacitance in the resultant capacitor, or acceptable capacitance at lower metal loadings in the dispersion. Such enhanced capacitance could result from densification of the fired capacitor electrodes (i.e., a reduction in the "laceyness" often observed in fired thick films of palladium particles). Also desirable are lower dissipation factors.